1. Technical Field
The present invention relates to a process facilitating hydrocarbon recovery from a subterranean formation and more specifically to a process preventing plugging of the formation and associated production wells.
2. Description of Related Art
Water is commonly injected into subterranean hydrocarbon-bearing formations by itself or as a component of miscible or immiscible displacement fluids to recover hydrocarbons therefrom. Injection water can be obtained from a number of sources including brine produced from the same formation, brine produced from remote formations, or sea water. All of these waters typically have a high ionic content relative to fresh water.
Some ions present in an injection water can benefit hydrocarbon production. For example, certain combinations of cations and anions, including K.sup.+, Na.sup.+, Cl.sup.-, Br.sup.-, and OH.sup.-, can stabilize clay to varying degrees in a formation susceptible to clay damage from swelling or particle migration.
However, other ions present in the injection water can produce harmful effects in situ. For example, divalent SO.sub.4.sup.- anions in the injection water are particularly problematic because SO.sub.4.sup.- forms salts with many naturally-occurring cations already present in the formation, such as Ba.sup.++. The resulting salts can be relatively insoluble at the formation temperatures and pressures. Consequently they precipitate out of solution in situ. Solubility of the salts further decreases as the injection water is produced to the surface with the hydrocarbons because of pressure and temperature decreases in the production well.
The precipitates of the insoluble salts accumulate in subterranean fluid passageways as crystalline structures which ultimately plug the passageways and reduce hydrocarbon production. The effects of plugging are most severe in passageways located in the formation near wellbores and in production wells where it is more difficult for the produced fluids to circumvent blocked passageways.
Prior art solutions to the problem of formation and production well plugging focus on preventing or inhibiting crystal formation in situ by supplementing the injection water with additives or backflowing a well with produced formation water containing additives. For example, ethylenediaminetetraacetic acid (EDTA) is a crystal modifier which can inhibit the in situ growth of crystals from insoluble salt precipitates. However, prior art processes are not totally satisfactory because the cost of chemical additives is significant and the cost escalates over the production life of the formation. Furthermore, the processes are ineffective without painstaking process control to ensure proper stochiometric concentration and in situ mixing of the additives.
An effective process for preventing plugging in a hydrocarbon-bearing formation and associated production wells by insoluble salts is needed which overcomes the drawbacks of the prior art. A process is needed which is relatively low cost and is relatively easy to control at the surface.